Light-emitting diodes (LEDs) are at the heart of solid-state lighting devices, which are gaining acceptance for use in a wide range of applications from traffic lights to large flat panel displays. Until now, the most common LED-based source of white light has been the so-called phosphor-converted LED (pcLED). A pcLED typically consists of a LED made from a single chip of a III-V semiconductor material such as indium gallium nitride (InGaN), and emitting light in a narrow wavelength range from about 450 nanometers (nm) to 470 nm. The light radiated in this wavelength range is perceived to have a blue color. A part of the blue light emitted by the LED is absorbed by a special material such as cerium-doped yttrium-aluminum-garnet ((Y1-aGda)3(Al1-bGab)5012:Ce3+), abbreviated as YAG:Ce. This special material is usually known as a phosphor, and it is embedded in an encapsulant transparent resin that surrounds the blue-emitting LED.
The phosphor causes down-conversion of the absorbed blue photons through a photoluminescence process to yield a light emission characterized by a broad spectrum that peaks around the 550-nm wavelength. This light is perceived to have a yellowish color. The portion of the blue light that is not absorbed by the YAG:Ce phosphor escapes to the outside and mixes with the yellow luminescent emission to generate white light. FIG. 1 shows a typical spectrum of the white light emission from a pcLED. The figure has been taken from U.S. Pat. No. 5,998,925 to Shimizu et al. The spectrum clearly shows the relatively narrowband intrinsic emission of the blue-emitting LED that peaks around the 450-nm wavelength and the broad luminescent emission that spans from about 500 nm to 700 nm.
As mentioned in U.S. Pat. No. 7,267,787 to Dong et al., the correlated color temperature of the overall white light emission from pcLEDs varies typically from 6000 K to 8000 K (cool white), while the color rendering index (CRI) of these light sources is in the range of 70 to 75. A CRI in this range results in a poor rendering of many colors, which often manifests as a lack of both deep green and red colors. The poor color rendering of white pcLEDs limited their use to some specific applications such as in flashlights, solar-powered lighting, and as light sources for energy-efficient backlighting for liquid-crystal televisions and computer displays. Owing to the ongoing advances in the development of high-brightness white LEDs, these devices are expected to take a larger part of high-power illumination systems. Examples of such illumination systems and their uses include streetlights, headlamps of car vehicles, domestic lighting, illumination for commercial buildings, and directed-area lighting for architectural purposes. LEDs are becoming more and more efficient at converting electrical power into light at a relatively low cost, and this makes them natural choices as energy efficient alternatives to standard lighting devices.
Semiconductor laser diodes, and more recently LEDs, have been used as light sources in lidar (LIght Detection And Ranging) systems, also commonly referred to simply as lidars. In addition to their traditional uses in remote-sensing applications and optical sounding of the atmosphere, lidars now find their way into various applications that range from level sensing of the top surface of liquids and materials stored in containers to adaptive cruise control (ACC) and collision-avoidance systems for car vehicles. The ever-growing optical power that can be radiated from white pcLEDs at a relatively low cost makes them promising candidates as light sources in cost-effective lidars.
Unfortunately, their use in lidars for high-accuracy optical detection and ranging of targets or obstacles located at close to medium range is plagued by some drawbacks. One of these relates to the relatively long decay time of the luminescent emission from the phosphor, which is typically about 60 nanoseconds (ns) for YAG:Ce material. As mentioned in U.S. Pat. Nos. 5,889,583 and 6,043,868, both to Dunne, LEDs would also have the inherent problem of a variable delay time between the drive current and the optical output, the delay being dependent on the current level and the junction temperature. The technology disclosed in both patents aims at obtaining steeper leading edges for the light pulses produced by a LED used as the light source of an optical rangefinder device. The method relies on an optional pre-biasing circuit that provides a reverse-bias signal to the LED prior to firing it. The improvement is intended to enable more accurate distance measurements with a LED-based optical rangefinder.
It could be very advantageous for several practical applications to combine into a single apparatus a conventional lighting system and a lidar instrument for optical detection and ranging at close to medium range. For example, significant reductions in the hardware complexity and manufacturing cost of ACC and collision-avoidance systems for car vehicles could be obtained by using the light emitted from a lighting system (here the car headlamps) already present in vehicles to perform the forward-looking optical detection and ranging (lidar) function required in the operation of these systems. Both lighting and lidar functions could be implemented by using a single light source like an assembly of white pcLEDs mounted in a car headlamp, and integrating proper drive electronics and data/signal processing means. The assembly of white pcLEDs could then be driven and commanded to perform a lidar function in a transparent fashion, i.e., without affecting the primary lighting function of the car headlamps.